In microelectronic fabrication, it is often desirable to form an electrical contact between a metal surface and a semiconductor surface. For example, microelectromechanical systems (MEMS) such as pressure sensors, accelerometers, yaw rate sensors, and micromotors may comprise a silicon mesa that is anodically bonded to an insulating substrate, such as glass, over a conductive trace. In such a design, the silicon mesa often operates as an anchor or support for a suspended structure such as a diaphragm, or microbeam, as is well known to those skilled in the art. The silicon mesa and conductive trace form an electrical contact which is preferably of low resistance, and ohmic in nature. Further, such an electrical contact should have minimum contact potential because of the sensitivity of many MEMS devices to electric charges or fields.
However, conventional techniques for forming low resistance, ohmic contacts between semiconductor surfaces and metal surfaces are often not satisfactory because they require several additional processing steps to prepare the semiconductor surface, or they produce inconsistent yields in terms of the electrical characteristics of the resulting contacts. For instance, one technique involves heavily doping the semiconductor substrate and then depositing a metal layer directly over the doped semiconductor surface. A sintering or annealing step is then performed to cause the diffusion of the metal layer into the semiconductor. The metal deposited on the semiconductor is essentially allowed to alloy slightly in order to form a better contact between the semiconductor surface and a metal surface. This technique is disfavored because, among other things, it requires several additional processing steps in order to prepare the semiconductor surface, which not only complicates the fabrication of the MEMS device, but also may increase the cost of the MEMS device.
An alternative technique is to directly bond the semiconductor substrate or surface to another substrate or surface over a metal trace so that the metal trace is pressed against the semiconductor substrate or surface. For example, if a semiconductor substrate (typically silicon) is anodically bonded to a glass substrate over a metal trace fabricated on the glass substrate, then the pressure of the contacting surface of the semiconductor substrate against the metal trace is relied upon to form the electrical contact. While it is generally recognized that heavily doped silicon pressed against a conductive trace may make an ohmic contact, it is also recognized that surface states and thin layers of oxide or other contaminants on the silicon surface may separate the silicon surface from the metal trace. The pressure of the anodic bond is generally not well controlled, and may not break through the layers of oxide or contaminants. Consequently, the resulting electrical contacts between the silicon and the metal trace generally have resistances that vary widely, and are typically high (e.g., from 100 .OMEGA. to greater than 10 M.OMEGA.). Further, the resulting contacts are often non-ohmic, that is, the voltage drop across the contact is not proportional to the current, but rather, has a rectifying characteristic and/or a voltage offset or potential built into the contact. As a result, the operation, reliability, and stability of the resulting MEMS devices may be adversely affected.
Therefore, an unresolved need exists in the industry for a low resistance, ohmic contact between a metal surface and a semiconductor surface without depositing a metal layer directly on the semiconductor surface and without requiring other extensive preparation of the semiconductor surface.